view output of already running processes in linux

Question

I have a process that is running in the background (sh script) and I wonder if it is possible to view the output of this process without having to interrupt it.

The process ran by some application otherwise I would have attached it to a screen for later viewing. It might take an hour to finish and i want to make sure it's running normally with no errors.

Answer 1

There is already an program that uses ptrace(2) in linux to do this, retty:

http://pasky.or.cz/dev/retty/

It works if your running program is already attached to a tty, I do not know if it will work if you run your program in background.

At least it may give some good hints. :)

You can probably retreive the exit code from the program using ptrace(2), otherwise just attach to the process using gdb -p <pid>, and it will be printed when the program dies.

You can also manipulate file descriptors using gdb:

(gdb) p close(1)
$1 = 0
(gdb) p creat("/tmp/stdout", 0600)
$2 = 1

http://etbe.coker.com.au/2008/02/27/redirecting-output-from-a-running-process/

Answer 2

You could try to hook into the /proc/[pid]/fd/[012] triple, but likely that won't work.

Next idea that pops to my mind is strace -p [pid], but you'll get "prittified" output. The possible solution is to strace yourself by writing a tiny program using ptrace(2) to hook into write(2) and writing the data somewhere. It will work but is not done in just a few seconds, especially if you're not used to C programming.

Unfortunately I can't think of a program that does precisely what you want, which is why I give you a hint of how to write it yourself. Good luck!